diff --git a/src/llamafactory/data/aligner.py b/src/llamafactory/data/aligner.py
index 299bdca3..ef70d75b 100644
--- a/src/llamafactory/data/aligner.py
+++ b/src/llamafactory/data/aligner.py
@@ -120,15 +120,15 @@ def convert_sharegpt(
     even_tags = (dataset_attr.assistant_tag, dataset_attr.function_tag)
     accept_tags = (odd_tags, even_tags)
     for i, messages in enumerate(examples[dataset_attr.messages]):
+        if len(messages) == 0:
+            continue
+
         if dataset_attr.system_tag and messages[0][dataset_attr.role_tag] == dataset_attr.system_tag:
             system = messages[0][dataset_attr.content_tag]
             messages = messages[1:]
         else:
             system = examples[dataset_attr.system][i] if dataset_attr.system else ""
 
-        if len(messages) == 0:
-            continue
-
         aligned_messages = []
         broken_data = False
         for turn_idx, message in enumerate(messages):